1. Field of the Invention
The present invention relates to a fire retardant for the protection of forest and rangelands which is durable against the effects of weather. More specifically, this invention relates to fire retarding compositions which can be applied to foliage and vegetation to hinder or prevent the occurrence of a fire rather than, as is customarily the case, being applied after a fire is underway.
2. Description of the Prior Art
Forest fires and brush fires annually cause the loss of millions of dollars worth of timber and other property. Huge damage occurs because usually by the time the fire is detected and before control measures can be applied, it is out of control in a condition known as "wildfire". Not only is the direct property loss due to such fires catastrophic, but associated soil erosion and watershed problems are also significant. It is important, therefore, to minimize and control the spread of forest fires whenever possible.
One of the simplest and best known of fire fighting techniques is to distribute water, thickened water, or water containing a chemical fire retardant over timber and other foliage (called the "fuel") in the path of a forest fire to retard advancement of the flame front. Various methods of distributing the water, including direct spraying and aerial dropping, have been employed. Aerial dropping is advantageous since areas which are not easily accessible may be thereby treated.
According to U.S. Pat. No. 3,553,128, issued Jan. 5, 1971, there are two types of fire retardants commonly employed in fighting forest, range, grass and brushland wildfires. These fire retardants are classified as "short-term" or "long-term" retardants. Short-term retardants, as defined in U.S. Pat. No. 3,553,128, rely primarily upon the water they contain to retard combustion. Long-term retardants contain, in addition to water, a water-soluble chemical that effectively retards flaming combustion for brief periods even after the water has evaporated.
At present, the most commonly used retardant chemicals are aqueous solutions of ammonium salts such as monoammonium orthophosphate, diammonium orthophosphate, ammonium sulfate and the like.
It is common practice to thicken both long-term and short-term retardants with additives in order to improve their capabilities with respect to placement or confinement on the target, adherence to the forest fuel surface, moisture barrier buildup between the fuel and flame, moisture retention, chemical retardant deposit on the fuel surface, and the like.
In the aerial application of liquid fire suppressing materials such as commonly undertaken using fixed wing aircraft, the prior art has recognized that fluids of low viscosity, when dropped from substantial elevation, tend to atomize and therefore cannot be relied upon to descend with sufficient concentration on specific target areas. It has been found that liquid fire suppressing materials having high viscosity, for example 1500 centipoises, when dropped from substantial elevations, tend to hold together such that these liquids can be confined to a specific target area and will descend thereupon with sufficient concentrations to be effective.
As pointed out in U.S. Pat. No. 3,196,108, issued July 20, 1965, most thickening agents for increasing the viscosity of fire retardant compositions are not compatible with the most effective fire suppressing chemicals available. Furthermore, it is stated therein that many of the fire suppressing chemicals which are compatible with some thickeners have such a low fire suppressing capacity that great amounts of these fire suppressing materials must be employed when used with certain compatible thickeners. It has also been found, according to U.S. Pat. No. 3,196,108, that many of the fire suppressing materials used are effective when wet but have little or no fire suppressing character when dry.
U.S. Pat. No. 3,334,045, issued Aug. 1, 1967, which discloses a fire retardant composition of the "long-term" type, points out that the maximum fire retardancy period under usual summer conditions is from about 2 to about 3 hours. The patentee concludes that it would not be feasible to prepare a fire break by aerial means much in advance of a forest fire. Having recognized the prior art limitations as of that point in time, the patentee in U.S. Pat. No. 3,334,045 disclosed a fire retardant composition said to have excellent cohesion for aerial drop application coupled with extended periods of fire retardation effectiveness. The fire retardant life, however, of the composition of U.S. Pat. No. 3,334,045 was said to be "as high as several weeks under ordinary summer conditions". This is still not sufficiently durable for extended protection.
Prior art methods and compositions for managing forest and grass fires, therefore, were largely directed to non-durable systems. Such systems and techniques were only compatible with the task of managing forest fires after the fire had started rather than attempting to prevent such fires in the first place through chemical means. Known "long-term" retardants usually become inactive following 1/4 inch (0.63 cm) of precipitation, according to reports published by the U.S. Department of Agriculture Forest Service.
Furthermore, most of the rheological investigations disclosed in the literature were directed to improving flow and placement properties of compositions dropped from fixed-wing aircraft, not from helicopters or dispensed from so-called "ground tankers". Ground tankers and, to some extent, helicopters, are found to be the most desirable dispensing vehicles in the treatment of high risk, high-value foliage and vegetation.
For those prior art workers who may have considered providing a weather-durable ("semipermanent") fire retardant for foliage, the ability to produce a suitable composition at an acceptable cost was considered a major obstacle. Such a weather-durable fire retardant composition, if available, could be applied to certain high-risk, high-value forest and rangeland areas as a means of hindering or preventing wildfires rather than the customary procedure of controlling an existing wildfire.
A weather-durable fire retardant having proper economics could find immediate utility along rail and highway rights-of-way, around logging slash burn areas, campsites, power lines, etc. To be effective, compositions containing such weather-durable fire retardants should be capable of withstanding from 2 to 6 inches (5.1 to 15.2 cm) of rain, snow, fog, dew and wind. Thus, the minimum weathering requirement to be met by such a durable fire retardant is 2 inches (5.1 cm) of rainfall per fire season. The expected primary ignition sources to be encountered are small in size and transitory, e.g., matchbook incendiary devices, hot metal fragments from railroad brake shoes, flaming carbon exhaust particles, and the like.
Canadian Pat. No. 942,137, issued Feb. 19, 1974, discloses the use of a water-dispersed latex emulsion binder containing a substantially water-insoluble ammonium polyphosphate, for preventing burning of vegetation along roads, rights-of-way, in campsites and public parks. Optionally, a plasticizer is disclosed as a film-forming aid for the latex emulsion. A stated advantage of the composition of Canadian Pat. No. 942,137 is the degree of permanence achieved through water-insolubility.
U.S. Pat. No. 3,345,289, issued Oct. 3, 1967, discloses an aqueous fire retardant composition containing diammonium phosphate and a vinyl compound-maleic anhydride copolymer as a thickener. The compositions disclosed therein were intended to remain longer in place than earlier known compositions. Extended weather-durability was not embraced by the disclosure of U.S. Pat. No. 3,345,289, hence the latter patent describes fire retardant compositions useful in managing existing wildfires rather than hindering the occurrence of wildfires in the first place.